1. Field of the Invention
The present invention relates to an electron beam exposure system which is capable of correcting deviation of the electron beam path which is caused by drift of the crossover position.
2. Description of the Prior Art
An electron beam exposure system with which the shape and size of the electron beam spot may be varied has recently received a lot of attention. In this exposure system, the electron beam is projected through a first electron beam shaping aperture to form a first aperture image which is, in turn, projected through a second electron beam shaping aperture, so that the electron beam spot formed by superposition of both aperture images is formed on a target. The superposition of the two aperture images may be varied by deflecting the first aperture image. Therefore, the shape and size of the electron beam spot may be varied according to the degree of deflection.
When the crossover position of the electron beam drifts in an electron beam exposure system of this type, the target may not be exposed with a desired electron beam spot, and a correct circuit pattern may not be formed. When the crossover position drifts in the direction perpendicular to the axis of the electron optical column of the system, the axis of the electron optical column and the electron beam path are misaligned, and the electron lens exposure system becomes incapable of correctly controlling the electron beam. An electron beam exposure system has been proposed which has a servo controller for correcting the drift of the crossover position in the direction perpendicular to the axis of the electron optical column. In this electron beam exposure system, which are a faraday cup arranged in a lens barrel, the current of the electron beam passing through an aperture disposed at the crossover position is detected by a faraday cup arranged in a optical column, and alignment coil, which are arranged at the other crossover position are servocontrolled so that the electron beam current thus measured is maximized, thereby maintaining the magnification of the crossover image at the position of the coil at substantially 1. However, with this alignment control, since the crossover image is not an enlarged image at the position of the aperture, fine position deviation, that is, axial drift, may not effectively be detected. As a result, a large position deviation may be included in the enlarged crossover image formed by a projecting lens arranged at a later stage, and a stable beam intensity may not be obtained. Furthermore, when performing servocontrol for alignment, since the deflection center for alignment does not coincide with the beam shaping aperture position, the beam shape and size may fluctuate. Since the alignment servo controller is constantly in operation, the deflecting system is adversely affected. Since the faraday cup for measuring the electron beam current is supported by an electrical insulative member in the electron optical column, the charging of the insulative member makes the electron beam unstable.
In addition to the drift of the crossover position as described above, there is another type of drift in the crossover point position in the direction of the axis of the electron optical column when the cathode of the electron gun, e.g., a LaB6 tip, is heated for a long time interval and its front end shape changes, or when the electric power supplied for heating the electron gun changes. Since a deflecting center of a scanning deflector is generally set at the crossover position, this type of drift gives rise to a change in the degree of deflection of the aperture image of the electron beam, and hence, in the error in the size of the electron beam spot of the shape- and size-contolled electron beam, and in a deviation in the electron beam path when the beam size varies. However, this type of electron beam exposure system did not involve any countermeasures to these problems. Therefore, the conventional electron beam exposure systems lacked precision and reliability in the shape and size of the electron beam spot.